Breast-feeding women may be at particularly high risk of folate deficiency due to the demands of pregnancy followed by the demands of breast-feeding. A cohort study will be conducted among 400 low-income postpartum Mexican-American women living on the US/Mexico border to determine the impact of breast-feeding on maternal folate levels. This study has two objectives: (a) to compare levels of serum folate, erythrocyte folate, vitamin B12 and homocysteine between women who breast feed their infants with those who give their infants formula, and (b) to determine folate dietary intake and use of folate supplements during postpartum period. Interviews and blood samples will be obtained during the last trimester of pregnancy and 6 times during the 24-month postpartum follow-up. In women of child-bearing ages, low levels of folate and high levels of homocysteine have been associated with an increased risk of having an infant with spina bifida and other birth defects. Mexican-American women have elevated rates of infants with spina bifida and other birth defects, suggesting that it is particularly important to study risk factors for folate deficiency in this ethnic group. There are little data evaluating folate status and vitamin use among postpartum, Hispanic women, particularly for recent immigrants. These women often have special needs in the postpartum period resulting from language difficulties and adjustment to a new culture.